1. Field of the Invention
This disclosure generally relates to power supplies, for example, fuel cell systems, and to electrical power storage devices, for example, batteries and/or ultracapacitors.
2. Description of the Related Art
Fuel cells are known in the art. Fuel cells electrochemically react a fuel stream comprising hydrogen and an oxidant stream comprising oxygen to generate an electric current. Fuel cell electric power plants have been employed in transportation, portable and stationary applications. Electric power plants employing fuel cells as the sole source of power may have several disadvantages relating to the time it takes for the fuel cells to produce full power, and their surge demand capacity, for example.
Fuel cell output is proportional to the amount of reactants supplied. On start-up, there is typically a delay until the fuel cells reach full operating power. For this reason, such power plants are inadequate for some applications because they are not “instant on”. One approach has been to keep the fuel cells in such systems continuously running, either supplying power to the load or in a low output “stand-by” mode. While this approach improves response time, it exacerbates hydrogen storage issues by significantly increasing hydrogen consumption. In addition, operational lifetime of the power plant may be adversely affected compared to systems where the power plant is operated intermittently.
Fuel cells can be damaged if the load requirements exceed their maximum output. Thus, in power plants solely employing fuel cells, the rated output of the fuel cell stack is generally matched to the expected peak load. In applications where transient load increases are significantly higher than normal load requirements, this necessitates a larger size and output fuel cell stack than required for normal operation in order to deal with surge demand. This, in turn, undesirably increases the cost of the power plant.
Electric power plants are also described that employ a battery electrically coupled in parallel with the fuel cell stack to provide additional current when the demand of the load exceeds the output of the fuel cell stack and to store current when the output of the fuel cell stack exceeds the demand of the load, as taught in commonly assigned pending U.S. patent application Ser. No. 10/017,470 entitled “Method and Apparatus for Controlling Voltage From a Fuel Cell System”; Ser. No. 10/017,462 entitled “Method and Apparatus for Multiple Mode Control of Voltage From a Fuel Cell System” and Ser. No. 10/017,461 entitled “Fuel Cell System Multiple Stage Voltage Control Method and Apparatus”, all filed Dec. 14, 2001. This approach addresses the “instant on” and surge capacity problems described, above.
Valve regulated lead acid (VRLA) batteries are most often employed for this purpose, as they are readily available and relatively inexpensive. However, VRLA batteries are large and heavy. They are temperature sensitive and require environmentally-controlled conditions for optimum performance. Environmental regulations relating to the storage and operation of VRLA batteries also add to increased costs. In certain applications, such as point-of-presence battery banks for back-up power and/or uninterruptible power supply systems (UPS), the use of VRLA batteries is less than desirable.
At the same time, batteries have some desirable characteristics. For example: they have the ability to accept charge only when their terminal voltage is above a threshold voltage; the rate of inrush current is limited during charging, particularly when charged from a completely discharged state; hot swapping a hybrid system incorporating a fuel cell and battery will not short out the DC bus.
It would be desirable to have a fuel cell power plant that employs an electrical storage device other than secondary batteries that avoids the undesirable characteristics of such batteries, while maintaining their desirable characteristics. The present invention addresses this problem and provides other related advantages.